There are two different formulations of the rigid connector, rigid and
flexible. In the flexible form, the rigid motion is prescribed only in an average sense, and instead an assumption about linearly distributed traction fields is used.
In 3D the situation is more complex. Six degrees of freedom, usually selected as three translations and three parameters for the rotation, are necessary. For finite rotations, however, any choice of three rotation parameters is singular at some specific set of angles. For this reason, a four-parameter quaternion representation is used for the rotations in COMSOL Multiphysics. Thus, each rigid connector in 3D actually has seven degrees of freedom, three for the translation and four for the rotation. The quaternion parameters are called
a,
b,
c, and
d. These four parameters are not independent, so an extra equation stating that the following relation is added:
Under pure rotation, a vector from the center of rotation (Xc) of the rigid connector to a point
X on the undeformed object is rotated into
where x is the new position of the point originally at
X. The displacement is by definition
The parameter a can be considered as measuring the rotation, while
b,
c, and
d can be interpreted as the orientation of the rotation vector. For small rotations, this relation simplifies to
The rotation vector is available as the variables thx_tag,
thy_tag, and
thz_tag. Here
tag is the tag of the
Rigid Connector node in the Model Builder tree.
Here u is the displacement field on the boundaries,
ur is the rigid body displacement as given by
Equation 3-110, and
Frs is the reaction force field (Lagrange multiplier).
The flexible formulation of the rigid connector is based on the weak expression of the constraint, Equation 3-111. In the original weak form, however, the reaction force field can have any distribution, in order to enforce the rigid body motion.
Here Fc (unit: N) and
Fd (unit: N/m) are two global vectors, each having three degrees of freedom in 3D.
Fc can be directly interpreted as the reaction force at the center of rotation.
Fd can be considered as a representation of the gradient of the reaction force field.
where R is the rotation matrix. In 2D,
Fc is a vector with two components, whereas
Fd is a scalar.
If the boundaries selected in the rigid connector are not contiguous, then each set of connected boundaries will have its own set of Fc and
Fd degrees of freedom. The center of rotation,
Xc, is then taken as the center of gravity for each individual group of boundaries.